He has not yet disclosed his stake in the company, describing it only as “substantial.” Already, about 15% of Dell’s shareholders, including Southeastern Asset Management and T. Rowe Price Group, have publicly stated their opposition to the current proposal. Still, another 30% or so of Dell’s remaining holders, excluding Mr. Dell and his 16% stake, are needed for a majority to defeat or approve the deal.

Southeastern provided a detailed analysis last month that argued that Dell is worth closer to $24 than $13.65. Some say that now, Mr. Icahn and his larger-than-life presence, bolstered by his many media appearances and forceful language, could serve to rally undecided voters against the company.

“Southeastern started everyone’s thinking with some very professional analyses of possible financial alternatives, and now Icahn has taken it a step further and presented his own very clear proposal of something investors can specifically compare with Dell’s proposal,” said Gary Lutin, chairman of The Shareholder Forum an organization that works with companies and investors, and has requested access to the information used by Dell’s board to evaluate the buyout deal.

Even so, the outcome of the battle for votes may not become clear for several months. Active investors such as Mr. Icahn and Southeastern only represent a small portion of Dell’s shareholder base. The largest block of Dell holders are the passive index mutual funds and exchange-traded funds that own the stock only because it is constituent of indices like the S&P 500. About 18% of Dell Inc. shares were owned by passive index funds at the end of last year, according to an analysis of the most recent filings by Ipreo, a capital markets data and advisory firm.